The summer 2010 severe weather season has been one of the more active severe weather seasons in several years. It has been a season with several devastating tornadoes, including the major outbreak of June 17 2010, in which more than 30 tornadoes occurred across our service area in a 4 hour period. At this "mid point" of the summer, the Grand Forks NWS has issued more tornado and severe thunderstorm warnings than in the past two years. But why has this summer been so stormy?
The answers are relatively complex as the correct combination of temperatures, moisture and wind patterns necessary to produce severe storms and tornadoes are equally complex. One overarching force behind this years severe weather actually began last fall, and is now fading away. A moderate El Niño pattern developed last fall, which helped to modulate our temperatures over the past winter. The El Niño of 2009/2010 has begun to fade, with the first vestiges of La Niña becoming apparent in the equatorial Pacific. Research has shown, and this year seems to bear out, that summer seasons following an El Niño have more severe weather across the plains. However, this research is ongoing, especially into the "why" and "how" the El Niño impacts the amount of severe weather in the plains.
Based on what we do know from the historical record, summers following an El Niño tend to have several key, large scale factors that influence severe weather. 1) Lower than normal surface pressure across the central and eastern United States as well as much of western Canada,and higher than average pressure in the U.S. inter-mountain west. This often results in an enhanced southerly low level & surface flow. 2) In the upper atmosphere there is often a blocking ridge of higher pressure in the Gulf of Alaska, and lower pressures over Greenland the the northeast U.S. This usually results in an anomalously fast, west to northwest flow aloft. 3) The Gulf of Mexico is usually warmer, which results in more moisture available in the atmosphere for transport into the plains. 4) There is more low level moisture transport than usual to the plains, possibly due to the warmer Gulf of Mexico.
During El Niño periods, there is an enhanced jet stream that often causes the southern and eastern U.S. to have more winter and spring storms. During the summer, this enhanced jet stream activity tends to migrate northward into the northern plains. From what we know about basic synoptic meteorology, more low level moisture, enhanced low level convergence, and increased directional shear from the lower to upper levels of the atmosphere all support enhanced severe weather. This has certainly been the case so far for this spring and summer period.
What about the rest of the summer? Based on the changes in the Pacific, El Niño is fading and La Niña is developing. Yet the atmosphere is still responding to the affects of last winters El Niño. Recent satellite imagery shows a very active series of storm systems in the Pacific, with a stronger than usual jet stream occurring. While the pattern will begin to change as we get closer to the end of the summer, there is the likelihood that an enhanced severe weather threat will persist well into August.